Officials say it could take days and "possibly weeks" to get Japan's nuclear emergency at the Fukushima Dai-ichi plant under control, even as crews work overtime to dump tons of seawater on the facility.

Earlier Thursday, the U.S. government issued a warning that the nuclear situation in Japan is deteriorating. They also informed American citizens to get at least 50 miles away from the damaged reactor at the plant.

The Japanese government is trying to cool the reactor by dumping water on it from helicopters before it releases more radiation.

Helicopters continued to dump seawater into holes on the roof of damaged reactor No. 3, hoping to cool the overheated uranium fuel rods.

A senior official with the U.N.'s nuclear safety agency said later Thursday there had been "no significant worsening" at the Fukushima Dai-ichi nuclear plant, but that the situation remained "very serious."

Wednesday, the Japanese considered this procedure to be too dangerous. However, U.S. officials insisted that something be done.

"We're reminded of how American leadership is critical to our closest allies," President Barack Obama said. "Even if those allies themselves are economically advanced and powerful, there are moments when they need our help."

Experts said the 180 emergency workers at the plant may be on a suicide mission.

There is hope that Tokyo Electric will be able to get several power lines, knocked out by the earthquake, back online. Such a move could restore the plant's cooling system and help ease the crisis.

In the meantime, the U.S. State Department has sent aircraft to help Americans evacuate from northeast Japan. Also, Tokyo's airport remains packed with people trying to leave the island nation.

One 15-year-old girl was sent by her parents to stay with family members in the U.S.

"So there was the earthquake and there's the radiation, and my parents kind of freaked out," the teenager said. "So they wanted me to come and go to my aunt's house, so I'll be safe."

The question of safety right now is confusing for many, with differing assessments from the U.S. and Japan. Though the U.S. warned its citizens to stay at least 50 miles away from the plant, the Japanese government has only advised a 12-mile safety zone.